Application of thin CaP coatings to orthopedic implants involves a series of conditions necessary for improving the performance of commercial plasma-sprayed coatings. Apart from inducing faster bone regeneration and bonding between the implant surface and the newly formed bone, thin CaP coatings should be well adherent, and they should not release particles that could damage other components of the implant device. Moreover, their elastic mod-ulus should have an intermediate value, between that of the substrate and that of the bone, to reduce the intrinsic residual stresses in the implantation area.
Several factors determine the mechanical performance of the PLD coatings.
Their morphology, either columnar or globular
Their structure, either crystalline or amorphous
Their thickness
Their interface with the substrate.
We have already described the processing conditions leading to the diverse morphologies and crystallinity of CaP coatings, and the factors affecting the interface composition and its thickness have been addressed. As for the thick-ness, in general it affects the mechanical performance of coatings owing to increases in stress accumulation with increasing thickness [91]. Thus, if bioac-tivity of a thin CaP coating is safeguarded by its composition, the coating has to be as thin as possible with regard to its mechanical properties.
5.5.1 Adhesive Bond Strength
When considering the bond strength of HA coatings, either plasma-sprayed or PLD thin coatings, as measured in a pullout experiment it should be noted that the bond strength for a typical plasma-sprayed coating in reality measures the bond strength of the adhesive resin to a grit-blasted titanium surface. This point becomes clear in Fig. 5.12, which shows an SEM cross section image of a typical commercial plasma-sprayed coating specified to be 50 mm thick. Because the adhesive resin penetrates the plasma-sprayed coating’s pores and cracks, if the modus of failure is cohesive the value measures the cohesive failure of an HA-resin composite. If the failure is adhesive, the measured value is a
combination of the real adhesion strength of the HA coating to the metal substrate and that of the resin to the metal surface.
Moreover, the plasma-sprayed coating needs a previous grit-blasting treat-ment performed with ceramic powder (usually Al2O3or SiC) to add a mechan-ical interlocking contribution to the coating–substrate bonding mechanism.
This grit-blasting treatment leads to surface contamination and to the eventual release of these highly abrasive particles. Figure 5.12 also shows an alumina particle embedded into the titanium and clearly demonstrates that parts of the plasma-sprayed coating are not bonded to the substrate because the resin is in contact with the metallic substrate.
PLD coatings are usually dense and pore-free, so the resin cannot penetrate it, as shown in Fig. 5.13. Thus, in the case of PLD coatings, the pullout test measures different strengths depending on the fracture site. If the fracture is Fig. 5.12 Cross-sectional view of a polystyrene (PS)-HA coating. Note the microcracks penetrating the coating surface and even reaching the substrate. A large crack along the coating–substrate interface can be clearly appreciated, as can an embedded alumina particle from the grit-blasting roughening step. From [43], with kind permission of Springer Science and Business Media
Fig. 5.13 Cross-sectional view of a PLD-HA-coated textured titanium (Ti) rod.
The substrate surface texture is perfectly followed by the coating, without cracks or spallation. From [43], with kind permission of Springer Science and Business Media
situated at the resin–coating interface, the measurement corresponds to the resin–coating bond strength. However, if the fracture takes place in the coating, the coating cohesive strength is obtained. Finally, only when the coating–
substrate interface is fractured does the measurement deliver the coating–
substrate bond strength. Hence, testing samples must be carefully examined after performing the pullout test for elucidating the causes for failure and for correct interpretation of the measurements.
Table 5.1 shows the tensile bond strength as reported for PLD coatings in the literature. Only four groups have described results with pullout testing: three of them on titanium or its alloys [30, 42, 43] and one on polymer substrates [28]. No pullout studies have been performed with KrF laser-produced coatings. When using the ArF laser, it becomes clear that the processing temperature plays a major role and that it should be as low as possible to mantain an interface between the coating and the substrate, which is free of brittle titanium oxide phases. The highest tensile bond strength value [43] was 58 MPa for crystalline HA coatings at 4558C and 1 to 2 mm thickness; and the fracture was at the adhesive resin in all cases, without spallation of the coating. This value is much better than the 30 to 40 MPa reported for 5008 to 6008C [30] and 10 mm thickness, where the fracture occurred predominantly in the coating; these results indicate that with these thicker films and higher temperatures the intrinsic strength of the HA coating itself played a more critical role than that of the coating–substrate interface. Zeng et al. [42] reported values of 21 to 31 MPa with the THG- and FHG-Nd:YAG lasers. Compared to the values obtained for the plasma-sprayed (32.5 MPa [92]) and heat-treated sputtered coatings (>53 MPa [17]), the PLD coatings had improved performance, probably owing to the fact that no thermal annealing is needed to achieve sufficient crystallinity, and thus it is possible to reduce to a minimum the amount of titanium oxide at the metal– coating interface.
5.5.2 Scratch Testing
Mechanical performance of the samples has been thoroughly evaluated by scratch testing by a number of authors [20, 44, 93–95]. In general, it has been
Table 5.1 Tensile bond strength of pulsed laser deposited HA coatings
Laser type
Tensile bond
strength [MPa] Substrate type Tsubstrate(8C) Reference
ArF >58 Ti, grade 3 455 [43]
3 Ti, grade 3 550 [95]
30–40 Ti, grade 2 500–600 [30]
9.8 HA/PI room [28]
6 HA/PTFE room [125]
Nd:YAG (THG, FHG)
21.1–31.1 Ti, grade 2 0–600 [42]
Plasma sprayed 32.5 Ti, 99.3% – [92]
found that the lower the processing temperature is the higher is the critical load.
The titanium oxide interface layer should be minimized to attain the best mechanical performance of the HA coatings [96]. However, comparison between the different results is difficult because critical loads are determined by acoustic emission and/or friction force correlation to SEM observations after the scratch experiment. Moreover, substrate roughness, spherical stylus material, and diameters are different; loading and scan velocities are not equal as well, so a direct comparison of the critical load values becomes meaningless (see section on mechanical evaluation). Notwithstanding this fact, general trends can be concluded based on each set of experiments.
When coated at 5758C [94] with the KrF laser, coatings fail under the scratch test by spallating laterally from the diamond tip, and the failure load increases as the thickness decreases. The thinner the coating, the stronger is the influence of the substrate on the scratch characteristics: The thinnest coating does not fail adhesively (but cohesively) as the thickest ones do, and it only deforms plasti-cally because the substrate is ductile. For such a thin film, the adhesion to the substrate overcomes its own cohesive strength. However, this thinnest coating is amorphous. The influence of the interface layer on the adhesion is well docu-mented for this same type of coatings. If deposited during an interval of only 7 minutes but held at the processing temperature for various annealing times in the water environment, the interface layer grows as described previously, and its critical load drops accordingly.
Comparable thickness dependence studies have been made with the 355 nm wavelength of the Nd:YAG laser [44]. All processing parameters were similar to those used with the KrF laser mentioned above, with the exception of the pulse duration, which in this case is much shorter (10 ns) and the larger wavelength, thus yielding a higher deposition rate (fivefold) and higher a-TCP content compared to those deposited with the KrF laser. This is probably the reason for obtaining critical loads, which are strikingly low (0.4 N for 0.4 mm thickness) compared to those obtained with the 248 nm wavelength, although the same thickness dependence trend was confirmed.
When all parameters but the temperature are kept constant, coatings produced by an ArF laser can be amorphous HA (2908C) or crystalline HA (4608C) [95]. The scratch tests for both samples indicate good coating adhesion. The amorphous coating fails cohesively, and the crystalline coating deforms plastically without detaching from the surface even at loads as high as 24.4 N, as can be seen in Fig. 5.14. Plate-like crystals of irregular shape, with sizes smaller than a few tens of nanometers, con-stitute this crystalline coating, with many defects at the grain boundaries.
On applying load, extended grain boundary shifting takes place, and the material deforms plastically, without any spallation or detachment of the coating. The combination of shorter wavelength (193 nm) and lower temperature of deposition (4608C) has resulted in the exceptional adhe-sion properties of these coatings compared to those obtained under other conditions.
5.5.3 Elastic Modulus and Hardness
Nanoindentation has been used to determine the elastic Young’s modulus E and the hardness H of amorphous and crystalline coatings. Table 5.2 lists the results reported by various groups. When using the ArF laser [95], the obtained H and E values are in agreement with those measured in the amorphous and crystalline parts of the plasma-sprayed coatings [97]. Moreover, they demon-strate the higher ductility of these coatings compared to the titanium subdemon-strate as their H/E ratio is lower, thus explaining the material spreading on the groove Fig. 5.14 Scratch testing of the crystalline CaP coating produced by ArF laser (Tsub= 4608C, 45 Pa, 1 J/cm2) highlighting the principal events along the scratch length. From [95], with permission from Elsevier
Table5.2Young’smodulusandhardnessofpulsedlaser-depositedHAcoatings LasertypeTsubstrate(8C)Young’smodulusE(GPa)HardnessH(GPa)H/E(mm)Reference KrFRT+annealed 500–600111 170(PLD) (UV-PLD)2.7 7(PLD) (UV-PLD)0.024 0.041[87] RT+annealed 500–600130(PLD)5Ar+implanted0.038[99] 500 600140 185(UV-PLD) (UV-PLD)4.5 7.20.032 0.039[98] ArF290 46093 74.4–107(amorphous) (crystalline)1.6 0.55–1.06(amorphous) (crystalline)0.018 0.006[95] CorticalBone7–30[126] Ti138.34.60.113[95] Plasmasprayed73 120(amorphous) (crystalline)[97] Dataforcorticalboneandplasmasprayedcoatingshavebeenincludedforcomparison
observed during the scratch testing. The E value of the crystalline coatings lies between those of Ti and cortical bone. Hence, by varying the substrate tem-perature during deposition, a coating with graded Young’s modulus can be easily obtained by this technique; thus, improved reduction of the intrinsic residual stresses in the implantation area can be envisaged.
Nelea et al. [87, 98] obtained higher values of E and H by in situ UV annealing for KrF laser deposition of HA coatings. Subsequent ion implanta-tion also increases the values for the elastic modulus and hardness [99].
However, the use of higher temperatures (5008C and 6008C) may deteriorate the interface when a TiN buffer layer is not used. Despite of the improved mechanical performance obtained with these in situ or ex situ treatments, the higher complexity of the process may compromise the industrial application of these technological modifications.
Advances in the development of instrumentation for nanoindentation allow us to determine the elastic modulus and hardness as a function of the indentation depth to measure the effect of a buffer layer made of low temperature (4608C) but crystalline HA [78] deposited at the metal–coating interface of an HA film grown at much higher temperature (6508C). For layers of 1 mm thickness, the 25 nm buffer layer acts as an efficient protective diffusion barrier for oxygen, preventing growth of the titanium oxide phases at the interface. However, hydrogen from the water atmosphere decomposition can still diffuse through the coating and pro-duce titanium hydride and stressed titanium at the interface. Depth-dependent nanoindentation has demonstrated that doubling the water flow can drag out the undesired hydrogen, and it can augment the interface E and H values to the original elastic modulus and hardness of the titanium substrate. During impact tests the fatigue resistance (cycles to failure) of the coatings produced with a buffer layer and high water flow could be raised to three to six times that of coatings produced at low water flow, indicating that the impact test can consti-tute a valuable tool for interface optimization.